1. Field of the Invention
The present invention relates to a pressure sensor, comprising a gas supply, and a sensor channel system arranged to be supplied with a gas by the gas supply and arranged to vent the gas via one or more openings comprising at least one measurement opening, and the sensor channel system being arranged to perform a measurement indicative of the pressure at the at least one measurement opening.
2. Background Information
Many automated manufacturing processes require the sensing of the distance between a manufacturing tool and the product or material surface being worked. In some situations, such as semiconductor lithography, the distance must be measured with accuracy approaching a nanometer.
The challenges associated with creating a proximity sensor of such accuracy are significant, particularly in the context of photolithography systems. In the photolithography context, in addition to being non-intrusive and having the ability to precisely detect very small distances, the proximity sensor can not introduce contaminants or come in contact with the work surface, typically a semiconductor wafer. Occurrence of either situation may significantly degrade or ruin the semiconductor quality.
Different types of proximity sensors are available to measure very small distances. Examples of proximity sensors include capacitance and optical gauges. These proximity sensors have serious shortcomings when used in lithographic projection systems because physical properties of materials deposited on wafers may impact the precision of these devices. For example, capacitance gauges, being dependent on the concentration of electric charges, can yield spurious proximity readings in locations where one type of material (e.g., metal) is concentrated. Another class of problems occurs when exotic wafers made of non-conductive and/or photosensitive materials, such as Gallium Arsenide (GaAs) and Indium Phosphide (InP), are used. In these cases, capacitance and optical gauges may provide spurious results.
U.S. Pat. No. 4,953,388, entitled Air Gauge Sensor, issued Sep. 4, 1990 to Andrew Barada (“'388 Patent”), and U.S. Pat. No. 4,550,592, entitled Pneumatic Gauging Circuit, issued Nov. 5, 1985 to Michel Deschape (“'592 Patent”), disclose an alternative approach to proximity sensing that uses an air gauge sensor. The '388 and '592 patents are incorporated by reference herein in their entireties. These sensors use a sensor channel system comprising a reference and measurement nozzles to emit an air flow onto reference and measurement surfaces and measure back pressure differences within the sensors to determine the distance between the measurement nozzle and the measurement surface.
Furthermore, principles of pneumatic gauging are discussed in Burrows, V. R., The Principles and Applications of Pneumatic Gauging, FWP Journal, October 1976, pp. 31-42, which is incorporated by reference herein in its entirety. An air gauge sensor is not vulnerable to concentrations of electric charges or electrical, optical and other physical properties of a wafer surface. Current semiconductor manufacturing, however, requires that proximity is gauged with high precision on the order of nanometers. Earlier versions of air gauge sensors, however, often do not meet today's requirements for precision in lithographic projection apparatuses.
One improvement that has been made to improve the precision of air gauge sensors is to ensure a stable flow from a gas supply by using a mass flow controller and a gas pressure regulator at the input to the mass flow controller. The mass flow controller dissipates heat and is mounted remotely from the sensor channel system with a supply tube between the mass flow controller and the air gauge sensor. However, the supply tube represents volume. The larger the volume of the supply tube, the slower the response of the air gauge sensor. Because of the dissipation of heat, the mass flow controller is usually placed in a cabinet far away from the wafer stage compartment of the lithographic projection apparatus. The wafer stage compartment is a compartment of the lithographic projection apparatus wherein wafers are illuminated with a patterned beam of radiation while being supported by a wafer stage.
Also, the mass flow from the mass flow controller depends on the pressure of the air on the output side of the mass flow controller. The pressure of the air on the output side depends on the pressure at the measurement opening, so that the accuracy of the air gauge sensor is impaired. To overcome this, an accumulator is positioned at the output side of the mass flow controller to stabilize the pressure at the output side of the mass flow controller. However, this further increases the volume and decreases the response of the air gauge sensor.
Therefore, what is need is a system and method to provide a pressure sensor with increased positioning possibilities.